Global warming is intensifying the California drought

20 August 2015

Dry grassland and oak landscape of the Coastal Mountain Range in central California, among the parts of California most heavily impacted by the current drought, taken in August 2015.Credit: Dominick McPeake

WASHINGTON, D.C. – Global warming has made the California drought worse, according to a new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

The latest findings add a new wrinkle to – and do not contradict – the idea that the record-breaking drought is a result of natural variations. The wrinkle is that in the absence of rising temperatures that dry up soils and plants, the current drought would be approximately 15 to 20 percent less severe. Within a few decades the water losses from rising temperatures will create a new arid ‘normal’ for California, according to the new study.

Because warmer temperatures drive water from the ground into the sky, temperature is an important part of the drought equation, according to A. Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory in New York and lead author of the new study. It’s not just about how much water falls, but also how much evaporates, he said.

Abnormally low lake level at Horseshoe Lake in the high-elevation Mammoth Lakes Basin in the Sierra Nevada Mountains, taken in June 2015.Credit: Jennifer Bernstein

“A lot of people think that the amount of rain that falls out of the sky is the only thing that matters,” Williams said. “But warming changes the baseline amount of water that’s available to us because it sends water back into the sky.”

The researchers looked at precipitation, temperature, humidity, wind and other factors in multiple sets of month-to-month climate records from 1901 to 2014. Over those 11 decades they found no trend in long-term rainfall. On the other hand, average temperatures have risen about 1.4 degrees Celsius (2.5 degrees Fahrenheit) over the 114-year period. So when the drought began in 2012, the dry air began pulling what little moisture there was from the landscape, according to the new study.

Because the data used in the study was very complex, the researchers came up with a range for the amount that temperature is contributing to the current drought. They report that the range is 8 to 27 percent. The most likely amount is in the middle of that range, from 15 to 20 percent, said Williams.

Dry grassland and oak landscape of the Coastal Mountain Range in central California, among the parts of California most heavily impacted by the current drought, taken in August 2015.Credit: Dominick McPeake

The long-term warming/drying trend for the state would be more than enough to cancel out any increases in rainfall that have been projected to come from global warming, according to the study’s authors. According to climate models, by the 2060s, California will essentially be in a state of semi-permanent drought, broken only by occasional very intense, short rainfall events.

That also applies to the potential El Niño moisture which many forecasters are hoping will reach California this winter. The rain could fool people into believing that the drought is over, said Williams. But in coming years the rainfall will be fighting the increased warmth, and people will have to adjust to a new normal, he added.

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